模板题，注意要先判断根的编号。
#include<cstdio> #include<algorithm> #include<cstring> using namespace std; const int maxn=1e5; int N; int head[maxn]; int tol; struct node { int u; int v; int next; }edge[maxn]; void addedge (int u,int v) { edge[tol].u=u; edge[tol].v=v; edge[tol].next=head[u]; head[u]=tol++; } int h[maxn]; int father[20][maxn]; int isRoot[maxn]; void dfs (int x) { for (int i=head[x];i!=-1;i=edge[i].next) { int v=edge[i].v; if (v==father[0][x]) continue; father[0][v]=x; h[v]=h[x]+1; dfs(v); } } int lca (int x,int y) { if (h[x]<h[y]) swap(x,y); for (int i=17;i>=0;i--) if (h[x]-h[y]>>i) x=father[i][x]; if (x==y) return x; for (int i=17;i>=0;i--) { if (father[i][x]!=father[i][y]) { x=father[i][x]; y=father[i][y]; } } return father[0][x]; } int main () { int T; scanf("%d",&T); while (T--) { scanf("%d",&N); memset(head,-1,sizeof(head)); memset(isRoot,0,sizeof(isRoot)); tol=0; memset(h,0,sizeof(h)); memset(father,0,sizeof(father)); for (int i=0;i<N-1;i++) { int u,v; scanf("%d%d",&u,&v); addedge(u,v); addedge(v,u); isRoot[v]=1; } int root; for (root=1;root<=N;root++) if (!isRoot[root]) break; dfs(root); for (int i=1;i<=17;i++) for (int j=1;j<=N;j++) father[i][j]=father[i-1][father[i-1][j]]; int x,y; scanf("%d%d",&x,&y); printf("%d ",lca(x,y)); } return 0; }